1. Field of the Invention
This invention relates to a fluorine resin coated structure of aluminum or an aluminum alloy.
2. Description of the Prior Art
As is well known, fluorine resins have the best thermal resistance, chemical resistance and electrical insulating properties among plastics, and, interestingly, possess "anti-stick" and low friction characteristics.
Fluorine resin coated structures of aluminum or an aluminum alloy (hereafter often merely "aluminum" for purposes of brevity), to which the present invention pertains, have rapidly gained acceptance in recent years as materials for cookware due to the useful anti-stick properties of fluorine resins. Fluorine resins, however, are difficult to bond, as will be appreciated from the fact that they have superior antistick properties. Various methods have been suggested to date for bonding fluorine resins, and the following four methods are now mainly in commercial use.
1. A "primer method" which comprises coating an aluminum substrate with a "primer" consisting of a dispersion or suspension of a fluorine resin having phosphoric acid or chromic acid added thereto to render a metal surface adhesive, drying and baking the coating to further render the metal surface adhesive, and then further coating a dispersion or suspension of a fluorine resin as a finishing layer, followed by drying and baking.
2. A "hard coat" method which comprises (1) forming a hard undercoat on the surface of an aluminum or an aluminum alloy substrate by (a) a flame spraying method involving spraying a hard metal or oxide powder such as alumina, nickel or chromium onto the surface after roughening the surface by, for example, sand blasting, or (b) a frit method involving coating a suspension containing a hard substance such as water glass or a ceramic or the aluminum substrate and baking it at high temperatures to adhere it to the aluminum substrate and therefore to provide raised and depressed portions on the surface, and then (2) performing the same primer method as in 1 above.
3. An etching method which comprises providing numerous fine raised and depressed portions by chemical etching which involves treating the surface of an aluminum or aluminum alloy substrate with, for example, a hydrochloric acid solution or by electrolytic etching which involves anodically treating the surface in a solution containing an electrolyte of a chloride using a direct current source, and then coating a dispersion or suspension of a fluorine resin on the etched surface, followed by baking.
4. A method which comprises forming an etched surface on the surface of aluminum or an aluminum alloy substrate by the same method as in 3 above, anodically oxidizing the aluminum surface with an aqueous solution containing at least one oxidized film-forming compound, such as an aqueous solution of sulfuric acid, to provide an oxidized aluminum coating on the etched surface, and then coating a dispersion of a fluorine resin on the oxide coating, followed by baking.
The fluorine resin coated structure obtained by the primer method suffers from a considerable deterioration in adhesive strength when exposed to hot water or hot oil. This defect poses a problem in using the structure in cookware. Since the primer contains chromic acid, the primer layer is colored dark. Thus, when only a fluorine resin is used in the finish layer, the dark color of the primer layer appears through the transparent fluorine resin layer, which makes the appearance of the cookware unpleasant. In order to avoid this, it is the general practice to cover the color of the undercoat by adding a pigment filler to the top coat. However, probably because the top coat contains substances other than the fluorine resin, the anti-stick property of the top coat tends to be reduced with use. Furthermore, the use of chromic acid is undesirable from the viewpoint of food sanitation when this structure is used in cookware.
The abrasion resistance of the coated structure obtained by method 1 in cookware is also a problem. Method 2 intends to increase the abrasion resistance of the fluorine resin by melt adhering a solid fine powder of a material such as metal or a ceramic to the aluminum surface.
Method 3 affords fine raised and depressed portions by etching the aluminum metal surface instead of forming a primer layer thereon, and thereby improves the adhesion of the fluorine resin to the metal surface. Probably because it utilizes mechanical adhesion, deterioration in adhesive strength hardly occurs upon exposure to hot water or hot oil, as compared with the primer method. Since no primer is required, no problems arise with regard to the color of the product or food sanitation. Furthermore, a filler such as a pigment is not used, and the fluorine resin alone can be coated. Accordingly, the anti-stick effect of the fluorine resin in cookware is hardly reduced, and this method is superior to the primer method. However, this method still does not enable one to solve the problem of adhesion between the fluorine resin and aluminum.
Method 4 is an improvement over method 3, and further improves the adhesion of the fluorine resin to aluminum, providing the best fluorine resin coated structure for cookware among these conventional methods.
However, the fluorine resin coated structure obtained by the last method, like that obtained by method 1, still has a problem with abrasion resistance. Although the fluorine resin has high mechanical strength, the fluorine resin coated surface of cookware undergoes heavy wear. This problem could be solved by melt adhering hard metal or ceramic as an undercoat layer. However, such a method has the defect of complicated production steps and high production cost, and adhesion of the fluorine resin to aluminum is not as satisfactory as in the case of method 1.